The navy, double-breasted jacket featuring gold buttons – with a price tag of £79.99 – was available in all sizes when fashion fans first recognised it.
But a few hours later, the item had completely sold out on the high street retailer's online store – just another example of "the Kate effect".
Kate had teamed the blazer with a Breton stripe top from Me + Em, navy skinny jeans and cork and suede wedges from Stuart Weitzman.
The casual attire was in stark contrast to the black, knee-length Jenny Packham dress, complete with silver beading in the shape of a fern to represent New Zealand's national emblem, that she sported the previous night for a state reception with the country's Prime Minister John Key at Government House.
Another item from Kate's tour wardrobe that has been a sell-out success is the Tory Burch black and white geometric print dress that she wore for Prince George's first ever official engagement.
The $450 creation was available on the Net-A-Porter website, but just hours later the a message on the site read, "Unfortunately this item has sold out. Add it to your wish list and we will tell you if more stock becomes available."
It looks like baby wear manufacturers can also look forward to "the George effect" after the set of dungarees he was wearing for the playdate became an instant hit.
William and Kate's son wore the £75 pair of "sailboat smocked dungarees" by British designer Rachel Riley over a white milo body t-shirt by Chelsea Clothing Company and Navy Alex Pre-Walkers shoes by Early Days.
Shortly after pictures surfaced of the tot's playdate, the small sizes had completely sold out on the London-based designer's website.